GB2393594A

GB2393594A - Mobile handset clock correction

Info

Publication number: GB2393594A
Application number: GB0222188A
Authority: GB
Inventors: Richard Ormson
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2002-09-24
Filing date: 2002-09-24
Publication date: 2004-03-31
Anticipated expiration: 2022-09-24
Also published as: GB2393594B; GB0222188D0

Abstract

A mobile telecommunications handset including a mobile telecommunications module and a GPS module has a mechanism for correcting an internal clock with respect to a telecommunications radio frequency. To correct for clock errors the telecommunications radio frequency is compared with the local clock and the differences between them are averaged over a predetermined period of time, and this averaged value is used to correct the internal clock. The averaging function served to correct for changes in the received telecommunications radio frequency caused by the Doppler shift, therefore providing an accurate internal clock even when the handset is in motion.

Description

- 1 MOBILE HANDSET CLOCK CORRECTION

FIELD OF INVENTION

The present invention relates to the correction of clock errors in mobile devices such as mobile telephones, PDAs 5 on the like, particularly devices including facility for positioning systems such as GPS.

BACKGROUND OF THE INVENTION

Mobile electronic devices are increasingly used for multi-

function purposes. A mobile telephone now incorporates 10 not just voice communication technology, but also features such as e-mail, Web access and GPS positioning. The boundary between a portable computer, personal data assistant (PDA) and mobile telephone is blurred, and all such devices are herein termed mobile communications handsets. In common to all mobile communications handsets is the need to provide increased functionality at reduced cost.

The components used must be of the right specification for

the intended purpose, and this is increasingly a problem 20 in multipurpose devices. We have appreciated in particular the problem of providing an accurate clock, as required for GPS positioning, in a multipurpose mobile communications handset.

SUMMARY OF THE INVENTION

25 The invention is defined in the claims to which reference is directed. Preferred features are set out in the dependent claims.

C' - 2 - In a broad aspect, the invention resides in measuring the frequency offset caused by relative movement of a mobile communications device in particular a handset and a radio service due to the Doppler effect and using this as a 5 factor in controlling a local clock with reference to the frequency source. This technique allows more accurate clock correction.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the invention will now be described, by 10 way of example only, and with reference to the accompanying figures, in which: Figure 1: shows the main physical components of a mobile communications handset embodying the invention; Figure 2: shows the functions performed by the components of Figure 1; DESCRIPTION OF A PREFERRED EMBODIMENT

The embodiment of the invention described is a mobile telephone handset using either GSM or UMTS protocols and incorporating GPS functionality. To make best use of 20 hardware resources, the communication and GPS functionality shares certain physical components. The main such physical components are shown in Figure 1.

A mobile communications handset 1 includes devices for user interaction, including a keypad 6, microphone 4 and :5 speaker 2, as well as a display 22. The main processes for operation of the handset are executed by a processor 12 which operates under instruction of code in memory 8 or SIM memory 18 in a SIM card 14 which itself includes a SIM processor 16.

- 3 - A separate radio unit 10 handles the transmission and reception of voice and data signals, and includes analogue RF circuitry for broadcast and reception, analogue to digital (ADC) and digital to analogue (DAC) converters and 5 a digital signal processor (DSP). The radio unit thus comprises all hard/software necessary to receive and convert between digital signals and RF signals.

A separate GPS module 5 of known type is also incorporated, and is controllable using the common keypad lo and display described above. Also, the GPS module shares common hardware, in particular the power source and clock.

The processes within the handset are governed by a clock or clocks which are used to generate analogue frequencies for the RF unit, and to generate digital clocks for the processor 12 and DSP in the radio unit 10. The clock accuracy required for GPS is higher than that required for voice telecommunications as will now be discussed.

The Global Position System (GPS) is a well-known system which uses broadcast pseudo random codes to allow 20 receivers to determine time differences, and hence relative positions, between a transmitter and receiver.

The transmitters are satellites orbiting the earth in known orbit paths whose position at any given time is accurately known. Using received signals from four such is satellites, a receiver can unambiguously determine its position using trigonometry to an accuracy dependent upon the repetition rate of the code, accuracy of components and other factors, such as the atmosphere and multipath reflections. 30 To increase accuracy, more than the minimum of four reference transmitters are usually tracked. There are around 24 satellites available for tracking in the GPS system, of which 8 are specified to be "visible" by a

- 4 receiver at any given time. In fact, GPS receivers typically include 12 channels to allow up to 12 satellites to be tracked at once.

GPS satellites transmit two L-Band signals which can be 5 used for positioning purposes. The reasoning behind transmitting using two different frequencies is so that errors introduced by ionospheric refraction can be eliminated. The signals, which are generated from a standard frequency 0 of 10.23 MHz, are L1 at 1575.42 MHz and L2 at 1227. 60 MHz and are often called the carriers.

The frequencies are generated from the fundamental satellite clock frequency of f0 = 10.23 MHz.

Since the carriers are pure sinusoids, they cannot be used 5 easily for instantaneous positioning purposes and therefore two binary codes are modulated onto them: the C/A (coarse acquisition) code and P (precise) code.

Also it is necessary to know the coordinates of the satellites and this information is sent within the 20 broadcast data message which is also modulated onto the carriers. The coarse/acquisition (CA) code was so named as it was originally designed as a coarse position measurement signal on its own, or as an acquisition code to assist in 2s looking onto the phase of the precise code. However, the CA code is now used generally both for acquisition and for position tracking, and so will be referred to simply as the CA code herein.

The C/A code is a pseudo random (PN) binary code (states 30 of 0 and 1) consisting of 1,023 elements, or chips, that

! - 5 - repeats itself every millisecond. The term pseudo random is used since the code is apparently random although it has been generated by means of a known process, hence the repeatability. 5 Due to the chipping rate (the rate at which each chip is modulated onto the carrier) of 1. 023Mbps, the chip length corresponds to approximately 300m in length and due to the code length, the ambiguity is approximately 300km - i.e. the complete C/A code pattern repeats itself every 300km lo between the receiver and the satellite.

The code is generated by means of a linear feedback register which is a hardware device representing a mathematical PRN algorithm.

The receiver needs to know the actual position of :5 satellites in addition to knowing its relative position to them, and for that reason a data message is broadcast.

The data message includes information describing the positions of the satellites and their health status.

The position calculation thus determines a relative 20 position based on a time difference between a received and a local CA code. In the GES system, the more accurate the clock that is used, the better the location fix that can be attained. In the worst case, a clock that is highly inaccurate will not allow the location to be determined at 25 all. To have a reasonable lock time (i.e. a small number of seconds in normal use) and good location accuracy (<50m) the clock needs an accuracy of better than 0.1 ppm.

This being an ambiguity in time of O.lx10-6s which is a distance ambiguity of the speed of light (3xlO3)x(O.lx10-6) 30 =30m. The process of correlating the received CA code with the local code for the first satellite is called "acquisition" and requires a high accuracy clock.

f - 6 Thereafter, when "tracking" the CA code signals the time can be derived from the received CA codes.

We have appreciated that a separate, high accuracy, clock could be used for a GPS module within a mobile 5 communications handset to provide the desired accuracy, but this would increase the cost of the unit and power consumption. Accordingly, the embodiment uses the existing clock hardware in a mobile telephone, but increases accuracy by compensating the existing clock.

lo The functions performed by the main components in a handset embodying the invention are shown in Figure 2. An AFC (Automatic Frequency Control) mechanism 30 operates to control the frequency of a clock 32 by measuring a frequency difference in comparison to a received broadcast signal, in particular, the signal is the GSM or UMTS carrier, and applying a correction to the clock as a feedback loop. The received carrier signal 34 from the RF radio is compared within the DSP 36 to a local clock using a comparison function 38. A frequency error value 40 is 20 derived and passed to the CPU 12. The CPU averages the error over an interval X in a burst averaging function 42 and applies this to a correction voltage determining function 44. The correction voltage value so determined is applied to a DAC 46 which provides a correction voltage 25 to the clock 32.

In summary, the AFC 30 compares the output of the local

crystal clock to the reference signals being received over the air. If there is a difference, then the local clock is adjusted to match that being received, typically, the 30 local clock is within 0.05 ppm of the clock received over the air. The clock signal that the base station sends over the air is of course highly accurate, so the frequency correction mechanism provides the handset with an accurate local clock. This inaccuracy is due to

- 7 - quantisation within the feedback system, noise, and averaging that is applied to smooth out large changes.

The AFC mechanism efficiently removes variations caused by a number of crystal factors, such as changes in the output 5 load, crystal ageing, temperature changes and supply voltage changes.

We have appreciated, however, that there is a difficulty if the handset is in motion. The received reference signal will be Doppler shifted, and this shift, using the lo AFC circuit, will cause the local clock to shift. Such a shift may be larger than the inherent crystal errors discussed above. Such an error in the clock would lead to inaccurate position measurements or no position measurement at all. The ratio of frequency offset to 15 frequency is the ratio of the relative speed of the handset to the speed of light (C). Thus a speed of 30 m/s (-lOOKm/h) will result in a change in clock frequency of 0.1 ppm.

We have devised, therefore, a mechanism to reduce the 20 Doppler effect on the clock signal measurement. This is achieved by appreciating that the Doppler changes occur much more rapidly than the other factors discussed, typically on timescales of less than a minute. In contrast crystal ageing occurs over a period of years, and 25 changes due to temperature variation occur over a timescale of say 10 minutes and the output load should not change significantly for those operating modes where GPS would be used.

We have also appreciated that Doppler changes will switch 30 from positive to negative values when changing from motion towards and away from a base station, and discontinuously when handing over from one cell to another in a cellular system. The higher the speed of travel, the larger the

- 8 Doppler shift and the more often changes from positive to negative Doppler values will occur.

To calculate and correct for the Doppler changes, a correction function 50 comprises a rolling averaging s function 52 for determining an average frequency error over a defined number of samples or time period which reports an error value 54. The rolling average of the frequency error over a period of say 5 minutes will tend to average out the Doppler errors discussed, without lo diminishing correction of the other errors discussed. The correction function 50, thereby determines the error correction to apply according to the following equation: (sample(t) + sample(t - x)+....+ sample(t - nx)) non_doppler_AFC(Time= t)= (n+ 1) The values for the interval between samples (x) and is averaging time (nx) are preferably x = 5 seconds and nx = 5 minutes, though other values would also be appropriate and may be stored in memory 8 or configurable depending upon prevailing conditions. The correction function 50 itself is preferably implemented within the CPU 12.

20 The rolling average figure gives the error correction to apply with sufficient accuracy such that GPS positioning is possible. At worst, the clock accuracy would be 0.05 ppm, but in practice can be better, the theoretical limit being the quantisation of the AFC loop which is typically 2s 0.004 ppm.

The clock correction unit 56 takes the error value 54 from which Doppler effects have been removed and applies the correction such that the GPS positioning measurements may be accurately made. This can be in either of two possible 30 arrangements: by applying the correction to the AFC

( - 9 - mechanism 30 to correct the clock for the whole handset; or by digitally correcting the clock for GPS calculation only. In the first arrangement, the clock correction unit 56 5 replaces the determining function 44 and applies an appropriate voltage to the crystal in the handset clock 32 during the GPS acquisition process. Whilst this arrangement is operable, we have appreciated two drawbacks. First, the crystal clock will take a finite lo time to settle so that GPS acquisition will not be able to initiate immediately, and second, the changes to the clock may cause the GSM/UMTS function to be inoperable.

The second arrangement of the clock correction unit 56 is preferred. In this arrangement, the clock correction unit 15 56 comprises a calculation within the DSP of the GPS module within the handset, leaving the AFC mechanism unchanged and so not varying the handset clock 32. The clock correction unit 56 deducts the rolling average correction from the currently applied AFC setting - the 20 difference representing the error due to Doppler shift.

The difference figure is supplied to the DSP within the GPS module. The DSP uses this figure to apply denotation (a known technique for compensating for inaccurate clocks) during decode of the GPS signals.

Claims

c - 10 CLAIMS

1. A mobile communications handset having a clock correction mechanism for correction of a local clock with respect to a mobile telecommunications radio s frequency comprising: a clock error measurement function arranged to receive a locally generated clock signal and a received mobile telecommunications RF signal and to determine a frequency difference between the lo locally generated clock signal and the received mobile telecommunications RF signal; - an averaging function arranged to receive the frequency difference and to determine an average frequency difference over a defined time period 15 of time scale sufficient to average out errors caused by Doppler changes in the radio frequency due to movement of the handset; and - a clock correction function arranged to receive the average frequency difference and to provide 20 a corrected clock signal.

2. A mobile communications handset according to claim 1, wherein the clock correction function comprises a correction circuit for adjusting the locally generated clock.

2s

3. A mobile communications handset according to claim 2, wherein the correction circuit comprises a voltage control circuit for adjusting the voltage applied to a clock crystal which provides the locally generated clock.

- 11

4. A mobile communications handset according to claim 1, wherein the clock correction function comprises a digital calculation function for correcting for the inaccuracy of the locally generated clock, without 5 varying the locally generated clock.

5. A mobile communications handset according to claim 4, wherein the clock correction function is implemented for a position calculation circuit.

6. A mobile communications handset according to any lo preceding claim, wherein the defined time period is determined by the amount of movement of the handset relative to the RF signal.

7. A mobile communications handset according to any preceding claim, wherein the defined time period is is of the order 10 minutes.

8. A mobile communications handset according to any preceding claim, wherein the RF signal is a carrier of the mobile telecommunications signal and the corrected clock signal is provided for a position 20 calculation module.

9. A mobile communications handset according to any preceding claim, wherein the averaging function is arranged to determine the average frequency difference according to the function: (sample(t) + sample(t - x) +....+ sample(t - nx)) as non_doppler_AFC(Time= i)= (n+ 1) where x the time interval between samples and nx is the time period over which averaging occurs.

c - 12

10. A mobile communications handset according to any preceding claim, wherein the handset is a mobile telephone with a positioning function.

11. A mobile communications handset according to any 5 preceding claim, wherein the handset is a mobile telephone with a module for telecommunications and a GPS module, and having a single clock source for the mobile communications module and the GPS module, wherein the clock correction function is arranged to lo provide an accurate clock signal for GPS acquisition.